Method and system of joining thick sheets of non-weldable material using ultrasonic joining

ABSTRACT

A method and system for joining relatively thick (i.e., at least 0.07 inch, or between 0.08 and 0.24 inch) sheets of non-weldable aluminum or other material using multiple passes of ultrasonic joining and multiple plugs to create a single larger sheet. Angles and profiles are machined onto edges of the sheets, the angles are overlapped to form a scarf joint, and ultrasonic joining is performed on both sides of the scarf joint, resulting in channels where the angles and profiles intersect. Plugs are extruded or otherwise created and positioned in the channels, and ultrasonic joining is used to join the plugs in the channels. Additional plugs are added until the last plugs are proud of the surface, and then the plugs are machined until the joint is flush with the surface. The plugs may be of the same or a different material as the sheets.

RELATED APPLICATION

The present U.S. non-provisional patent application is a continuationand claims priority benefit of an earlier-field patent applicationhaving the same title, Ser. No. 15/717,197, filed Sep. 27, 2017. Theentire content of the identified earlier-filed application isincorporated by reference into the present application as if fully setforth herein.

FIELD

The present invention relates to methods of joining sheets ofnon-weldable materials, and more particularly, embodiments concern amethod of joining relatively thick sheets of non-weldable aluminum andother materials using multiple passes of ultrasonic joining and multipleplugs to create a single larger sheet.

BACKGROUND

In the manufacture of aircraft, ships, buildings, and other structuresit is often desirable to use large sheets of aluminum or other materialsfor, e.g., exterior surfaces, or “skins.” Using larger sheets ratherthan riveting together multiple smaller sheets both reduces the numberof fasteners and improves the performance of the structure. However,suppliers of aluminum sheets are limited in the sizes they can produceby milling technology. Thus, there is a need to join aluminum sheetstogether with base material-like properties.

Conventional welding is not suitable for use on high performancealuminum alloys (i.e., 2XXX and 7XXX) as it results in a joint having aheat-affected zone with changed heat treatment and corrosion issues.Friction stir welding can join aluminum, but it results in a stir zonewith varying metallurgical properties, and it also creates aheat-affected zone that reduces performance.

Ultrasonic joining is a joining technique which causes dynamicrecrystallization of new grains to form an interface. Ultrasonic joiningcan join aluminum but is limited in that the energy required to causedynamic recrystallization must travel through the material, and as thematerial becomes thicker, the energy requirement increasesexponentially. At some point, the energy required to cause the dynamicrecrystallization damages the exterior, which imposes a limit on thethickness, approximately 0.04 inch, that can be joined. Some work hasbeen done to successfully join sheets up to 0.07 inch thick by usingultrasonic joining on both sides of a scarf joint. However, ultrasonicjoining has not been used to join sheets between 0.08 inch and 0.24 inchthick, which would be useful in the aircraft manufacturing industry.

This background discussion is intended to provide information related tothe present invention which is not necessarily prior art.

SUMMARY

Embodiments address the above-described and other problems by providinga method of joining relatively thick sheets (i.e., at least 0.07 inch,or between 0.08 and 0.24 inch) of non-weldable aluminum using multiplepasses of ultrasonic joining and multiple plugs to create a singlelarger sheet.

In a first embodiment of the present invention, a method is provided forjoining first and second sheets of a non-weldable material having athickness of at least 0.07 inch to create a single larger sheet having afirst side and a second side. The method may broadly comprise thefollowing. For each sheet of the first and second sheets, an angle maybe cut on a first side of an edge of the sheet, and a profile may bemachined on a second side of the edge of the sheet. The angles on thefirst sides of both sheets may be overlapped, and ultrasonic joining maybe performed to join the first and second sheets together by a scarfjoint to form the single larger sheet, such that the angle of each sheetintersects the profile of the other sheet to create a first channel inthe first side and a second channel in the second side of the singlelarger sheet. For each channel of the first and second channels, atleast one plug may be positioned in the channel, with the plug having athickness of between 0.01 inch and 0.04 inch, and ultrasonic joining maybe performed to join the plug in the channel. A last plug may bemachined flush with the surface of the single larger sheet.

Various implementations of the first embodiment may include any one ormore of the following additional features. The first and second sheetsof the non-weldable material may each have a thickness of between 0.08inch and 0.24 inch. The non-weldable material may be a non-weldablealuminum material. Each plug may be of a different non-weldable materialthan each sheet. For each channel, at least one additional plug may bepositioned over an initial plug in the channel, and ultrasonic joiningmay be performed to join the additional plug in the channel until a lastadditional plug is proud of a surface of the single larger sheet. Foreach channel, the method may further include machining the initial plugultrasonically joined in the channel in preparation for joining theadditional plug over the initial plug. For each channel, there may be atleast one intermediate additional plug prior to the last additionalplug, and the method may further include machining the intermediateadditional plug joined in the channel in preparation for joining thelast additional plug over the intermediate additional plug. For eachchannel, positioning the plug in the channel may involve extruding andcutting the plug from a spool of stock material. The method may furtherinclude performing non-destructive inspection on the single larger sheetto determine one or more characteristics of the joining.

In a second embodiment of the present invention, a system is providedfor joining first and second sheets of a non-weldable material having athickness of at least 0.07 inch to create a single larger sheet having afirst side and a second side. The system may broadly comprise, for eachsheet of the first and second sheets, a cutting head, a first machininghead, and a first ultrasonic joining head. The cutting head may beconfigured to cut an angle on a first side of an edge of the sheet sothat the first and second sheets can be overlapped to form a scarf jointhaving first and second sides. The first machining head may beconfigured to machine a profile on a second side of the edge of thesheet. The first ultrasonic joining head may be configured to performultrasonic joining on the first and second sides of the scarf joint tojoin together the first and second sheets to form the single largersheet, such that the angle of each sheet intersects the profile of theother sheet to create a first channel in the first side and a secondchannel in the second side of the single larger sheet. The system mayfurther broadly comprise, for each channel of the first and secondchannels, a plug dispenser, a second ultrasonic joining head, and asecond machining head. The plug dispenser may be configured to positionat least one plug in the channel, with the plug having a thickness ofbetween 0.01 inch and 0.04 inch. The second ultrasonic joining head maybe configured to perform ultrasonic joining to join the plug in thechannel. The second machining head may be configured to machine a lastplug flush with the surface of the single larger sheet.

Various implementations of the second embodiment may include any one ormore of the following additional features. The first and second sheetsof the non-weldable material may each have a thickness of between 0.08inch and 0.24 inch. The non-weldable material may be a non-weldablealuminum material. Each plug may be of a different non-weldable materialthan each sheet. For each channel, the plug dispenser may be furtherconfigured to position at least one additional plug over an initial plugin the channel, and the second ultrasonic joining head may be furtherconfigured to perform ultrasonic joining to join the at least oneadditional plug in the channel until a last additional plug is proud ofthe surface of the single larger sheet. For each channel, the system mayfurther include, an intermediate machining head configured to machinethe initial plug joined in the channel in preparation for joining the atleast one additional plug over the initial plug. For each channel, theremay be at least one intermediate additional plug prior to the lastadditional plug, and the system may further include an intermediatemachining head configured to machine the intermediate additional plugjoined in the channel in preparation for joining the last additionalplug over the intermediate additional plug. For each channel, the plugdispenser may extrude and cut the plug from a spool of stock material.The system may further include a non-destructive inspection deviceconfigured to perform non-destructive inspection of the single largersheet to determine one or more characteristics of the joining.

This summary is not intended to identify essential features of thepresent invention, and is not intended to be used to limit the scope ofthe claims. These and other aspects of the present invention aredescribed below in greater detail.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a flowchart of steps in an embodiment of a method of joiningrelatively thick sheets of non-weldable material using ultrasonicjoining;

FIG. 2 is a fragmentary cross-sectional side elevation view of first andsecond sheets to be joined using the method of FIG. 1;

FIG. 3 is a fragmentary cross-sectional side elevation view of theresult of an angle cutting step of the method of FIG. 1 on the sheets ofFIG. 2;

FIG. 4 is a fragmentary cross-sectional side elevation view of theresult of a profile machining step of the method of FIG. 1;

FIG. 5 is a fragmentary cross-sectional side elevation view of theresult of an ultrasonic joining step of the method of FIG. 1;

FIG. 6 is a fragmentary cross-sectional side elevation view of theresult of an initial plug installation step of the method of FIG. 1;

FIG. 7 is a fragmentary cross-sectional side elevation view of theresult of a final plug installation step of the method of FIG. 1;

FIG. 8 is a fragmentary cross-sectional side elevation view of theresult of a final machining step of the method of FIG. 1, showing thefinished joint; and

FIG. 9 is a diagram of a system for joining relatively thick sheets ofnon-weldable material using ultrasonic joining.

The figures are not intended to limit the present invention to thespecific embodiments they depict. The drawings are not necessarily toscale.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying figures. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thosewith ordinary skill in the art to practice the invention. Otherembodiments may be utilized and changes may be made without departingfrom the scope of the claims. The following description is, therefore,not limiting. The scope of the present invention is defined only by theappended claims, along with the full scope of equivalents to which suchclaims are entitled.

In this description, references to “one embodiment,” “an embodiment,” or“embodiments” mean that the feature or features referred to are includedin at least one embodiment of the invention. Separate references to “oneembodiment,” “an embodiment,” or “embodiments” in this description donot necessarily refer to the same embodiment and are not mutuallyexclusive unless so stated. Specifically, a feature, component, action,step, etc. described in one embodiment may also be included in otherembodiments, but is not necessarily included. Thus, particularimplementations of the present invention can include a variety ofcombinations and/or integrations of the embodiments described herein.

Broadly characterized, embodiments provide a method and system ofjoining relatively thick sheets of non-weldable aluminum and othermaterials using multiple passes of ultrasonic joining and multiple plugsto create a single larger sheet. In more detail, the method and systememploys a multi-step process to overcome existing limitations on thethicknesses of materials amenable to ultrasonic joining, and allows forjoining together sheets of non-weldable aluminum or other materialhaving thicknesses of greater than 0.07 inch, or greater than 0.08 inch,or between 0.08 inch and 0.24 inch.

Referring to FIGS. 1 and 2-8, an embodiment of a method 20 of joiningfirst and second relatively thick sheets of non-weldable aluminum orother material 120,122, as seen in FIG. 2, using ultrasonic joining isshown. The sheets 120,122 are shown having X, Y, and Z axes, with thesheets being joined along the X axis. The method 20 may broadly comprisethe following.

A first angle 124 may be cut onto a first or inner side of an edge ofthe first sheet 120, and a second angle 126 may be cut onto acorresponding first or inner side of the second sheet 122, as shown in22 and seen in FIG. 3. The purpose of the first and second angles124,126 may be to facilitate forming a scarf joint, so the angles124,126 may be substantially identical, and each angle 124,126 may beless than 45 degrees, or less than 20 degrees, though the actual slopemay depend on such factors as the thickness of the sheets 120,122 andthe ability of the ultrasonic joining to penetrate the material. In oneimplementation, the first and second angles 124,126 may have asubstantially planar contour (as shown in the figures), while in anotherimplementation, the first and second angles 124,126 may have asubstantially non-planar contour, such as a curved or a stepped contour,or, more broadly, a simple or a complex contour.

A first profile 128 may be machined on a second or outer side of theedge of the first sheet 120, and a second profile 130 may be machined ona corresponding second or outer edge of the second sheet 122, as shownin 24 and seen in FIG. 4. The first and second profiles 128,130 may besubstantially identical, and each profile 128,130 may be substantiallyangular and slope toward the edge of the sheet 120,122. In oneimplementation, the first and second profiles 128,130 may have asubstantially planar contour (as shown in the figures), while in anotherimplementation, the first and second profiles 128,130 may have asubstantially non-planar contour, such as a curved or a stepped contour,or, more broadly, a simple or a complex contour.

The first and second sheets 120,122 may be arranged so that the firstand second angles 124,126 overlap, forming a scarf joint 132, andultrasonic joining may be performed on both sides of the scarf joint 132to join the first and second sheets 120,122 together, as shown in 26 andseen in FIG. 5. On one side of the scarf joint 132, the first profile128 may intersect the second angle 126 to form a first channel 134,while on the other side of the scarf joint 132, the second profile 130may intersect the first angle 124 to form a second channel 136. Asneeded or desired, the first and second channels 134,136 may be machinedin preparation for joining the first and second plugs, as shown in 28.

A first plug 138 may be created and positioned in the first channel 134,and ultrasonic joining may be performed to join the first plug 138 inthe first channel 134, and a second plug 140 may be created andpositioned in the second channel 136, and ultrasonic joining may beperformed to join the second plug 140 in the second channel 136, asshown in 30 and seen in FIG. 6. As needed or desired, the first andsecond plugs 138,140 ultrasonically joined in the first and secondchannels 134,136 may be machined in preparation for joining additionalplugs, as shown in 32.

If the sheets 120,122 are not substantially thicker than 0.07 inch(i.e., not greater than 0.09 inch thick, or not greater than 0.08 inchthick), then only the first and second plugs 138,140 may be needed tocomplete the complete the joint. However, as needed or desired, two ormore additional plugs 142,144 may be created and positioned in the firstand second channels 134,136 over the previously installed plugs, andultrasonic joining may be performed on the two or more additional plugs142,144 to join the two or more additional plugs 142,144 in the firstand second channels 134,136 until at least first and second lastadditional plugs are proud of the surfaces of the first and secondsheets 120,122, as shown in 34 and seen in FIG. 7. As needed or desired,any of the two or more additional plugs 142,144 prior to the first andsecond last additional plugs ultrasonically joined in the first andsecond channels 134,136 may be machined in preparation for joiningadditional plugs, as shown in 36. At least the first and second lastadditional plugs (or as mentioned, the first and second plugs 138,140 ifthey are the only and therefore last plugs) may be machined until theyare flush with the surfaces of the first and second sheets 120,122(i.e., until the joint is the same thickness as the base material) toproduce the finished joint 146, as shown in 38 and seen in FIG. 8.

The first and second plugs 138,140 and/or the two or more additionalplugs 142,144 may each have a thickness of between 0.01 inch and 0.04inch. The first and second plugs 138,140 and/or the two or moreadditional plugs 142,144 may be of the same or a different material asthe sheets 120,122. For example, if the sheets 120,122 are a type ofaluminum, then the plugs 138,140,142,144 may be the same or a differenttype of aluminum, or they may be steel or another body-centered cubicmaterial, or they may be titanium or another hexagonal close-packedmaterial. In one implementation, the plugs 138,140,142,144 may be of atougher material than the sheets 120,122 in order to further resistcracking. The first and second plugs 138,140 may present inner surfaceshaving contours that engage or otherwise complement the contours of theangles 124,126 and/or profiles 128,130, and may present outer surfaceshaving contours that may be similarly planar or non-planar. Similarly,the two or more additional plugs 142,144 may present inner surfaceshaving contours that engage or otherwise complement the contours of theouter surfaces of the first and second plugs 124,126, and may presentouter surfaces having contours that may be similarly planar ornon-planar.

In one implementation, the plugs 138,140,142,144 may be dispensed aspre-formed plug material into the channels 134,136. In anotherimplementation, the plugs 138,140,142,144 may be formed from plugmaterial as they are dispensed into the channels 134,136. In the latterimplementation, the plug material may be spooled wire other feedstockmaterial.

At least the finished joint 146 or the resulting single larger sheet maybe tested, evaluated, or otherwise inspected using a non-destructiveinspection technique, as shown in 40, to determine whether the joint 146is satisfactory for the intended purpose of the sheet.

Thus, the method 20 may be used to create relatively thick sheets ofnon-weldable aluminum of substantially any size.

Referring also to FIG. 9, an embodiment of a system 220 is shown forjoining first and second relatively thick sheets of non-weldablealuminum or other material 120,122 using ultrasonic joining. The system220 may implement some or all of the aspects of the method 20 describedabove, and the corresponding method steps are referenced in thefollowing description of the system 220. The system 220 may broadlycomprise first and second end effectors 222,224, with each end effectorincluding a first machining head 226,228; a first ultrasonic joininghead 230,232; a second machining head 234,236; a plug dispenser 238,240;a second ultrasonic joining head 242,244; a third machining head246,248; and first and second NDI devices 250,252.

The first and second end effectors 222,224 may be moveable along thelength of the edges (i.e., the X axis) of the sheets of material 120,122to be joined. The end effectors 222,224 may be moved by robotic devices.In an alternative implementation, the sheets 120,122 may be movedrelative to the end effectors 222,224 which may remain stationary. Thefirst end effector 222 may be positioned and configured to act on afirst side of the sheets 120,122, and the second end effector 224 may bepositioned and configured opposite the first end effector 222 andconfigured to act on a second side of the sheets 120,122. The second endeffector 224 may be further configured to react to loads created by thefirst end effector 222.

As illustrated, prior to the sheets 120,122 entering the system 220, afirst angle 124 may be cut onto a first or inner side of an edge of thefirst sheet 120, and a second angle 126 may be cut onto a correspondingfirst or inner side of the second sheet 122 (corresponding to step 22 ofFIG. 1, and seen in FIG. 3), in order to facilitate forming a scarfjoint. Alternatively, the system 220 may include first and secondcutting heads configured to cut the angles 124,126.

The first machining heads 226,228 may be configured to machine a firstprofile 128 on a second or outer side of the edge of the first sheet120, and to machine a second profile 130 on a corresponding second orouter edge of the second sheet 122 (corresponding to step 24, and seenin FIG. 4).

The first and second sheets 120,122 may be arranged so that the firstand second angles 124,126 overlap, forming a scarf joint 132, and thefirst ultrasonic joining head 230,232 may be configured to performultrasonic joining on both sides of the this scarf joint 132 to join thefirst and second sheets 120,122 together (corresponding to step 26, andseen in FIG. 5). On one side of the scarf joint 132, the first profile128 may intersect the second angle 126 to form a first channel 134,while on the other side of the scarf joint 132, the second profile 130may intersect the first angle 124 to form a second channel 136. Asneeded or desired, the second machining head 234,236 may be configuredto machine the first and second channels 134,136 in preparation forjoining the first and second plugs (corresponding to step 28).

The plug dispenser 238,240 may position a first plug 138 in the firstchannel 134, and may position a second plug 140 in the second channel136 (corresponding to a first part of step 30, and seen in FIG. 6). Inone implementation, the dispenser 238,240 may dispense pre-formed plugmaterial into the channels 134,136. In another implementation, thedispenser 238,240 may form the plug material as it is dispensed. In thelatter implementation, plug material may be in the form of spooled wireor other feedstock.

The second ultrasonic joining head 242,244 may be configured to performultrasonic joining to join the first plug 138 in the first channel 134,and the second plug 140 in the second channel 136 (corresponding to asecond part of step 30, and seen in FIG. 6). As needed or desired, thesecond machining head 234,236 may be configured to machine the first andsecond plugs 138,140 in preparation for joining additional plugs(corresponding to step 32).

As needed or desired, the plug dispenser 238,240 may position two ormore additional plugs 142,144 in the first and second channels 134,136over the previously installed plugs, and the second ultrasonic joininghead 242,244 may perform ultrasonic joining to join the two or moreadditional plugs 142,144 in the first and second channels 134,136 untilat least first and second last additional plugs are proud of thesurfaces of the first and second sheets 120,122 (corresponding to step34, and seen in FIG. 7. As needed or desired, the second machining head234,236 may be configured to machine any two or more additional plugs142,144 prior to the first and second last additional plugsultrasonically joined in the first and second channels 134,136 inpreparation for joining additional plugs (corresponding to step 36).

The third machining head 246,248 may be configured to machine at leastthe first and second last additional plugs until they are flush with thesurfaces of the first and second sheets 120,122 (i.e., until the jointis the same thickness as the base material) to produce the finishedjoint 146 (corresponding to step 38, and seen in FIG. 8).

The NDI device 250 may be configured to test, evaluate, or otherwiseinspect at least the finished joint 146 of the resulting single largersheet using a non-destructive inspection technique (corresponding tostep 40), to determine whether the joint 146 is satisfactory for theintended purpose of the sheet.

Thus, the system 220 may be used to create relatively thick sheets ofnon-weldable aluminum of substantially any size.

Although the invention has been described with reference to the one ormore embodiments illustrated in the figures, it is understood thatequivalents may be employed and substitutions made herein withoutdeparting from the scope of the invention as recited in the claims.

Having thus described one or more embodiments of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. A method of joining first and second sheets of amaterial to create a single larger sheet having a first side and asecond side, the method comprising: for each sheet of the first andsecond sheets— cutting an angle on a first side of an edge of the sheet,and machining a profile in a second side of the edge of the sheet;overlapping the angles on the first sides of both sheets; performingultrasonic joining to join the first and second sheets together by ascarf joint to form the single larger sheet, such that the angle of eachsheet intersects the profile of the other sheet to create a firstchannel in the first side and a second channel in the second side of thesingle larger sheet; and ultrasonic joining one or more plugs in each ofthe first and second channels.
 2. The method of claim 1, wherein each ofthe first and second sheets of the material have a thickness of at least0.07 inch.
 3. The method of claim 1, wherein the material is an aluminummaterial.
 4. The method of claim 1, wherein each plug has a thickness ofbetween 0.01 inch and 0.04 inch.
 5. The method of claim 1, wherein eachof the plugs are of a different material than the first and secondsheets.
 6. The method of claim 1, further including, for each channel ofthe first and second channels, positioning at least one additional plugover an initial plug in the channel, and performing ultrasonic joiningto join each additional plug in the channel until the last plug is proudof a surface of the larger sheet.
 7. The method of claim 6, furtherincluding, for each channel of the first and second channels, machiningthe initial plug ultrasonically joined in the channel in preparation forjoining the additional plug over the initial plug.
 8. The method ofclaim 7, wherein, for each channel of the first and second channels,there is at least one intermediate additional plug prior to the lastplug, and further including, for each channel of the first and secondchannels, machining the intermediate additional plug joined in thechannel in preparation for joining the last plug over the intermediateadditional plug.
 9. The method of claim 1, further including machining alast plug of the one or more plugs flush with the respective side of thesingle larger sheet.
 10. The method of claim 1, further includingperforming non-destructive inspection on the larger sheet to determine acharacteristic of the joining.
 11. A method of joining first and secondsheets of a material to create a larger sheet having a first side and asecond side, the method comprising: for each sheet of the first andsecond sheets— cutting an angle on a first side of an edge of the sheet,and machining a profile in a second side of the edge of the sheet;overlapping the angles on the first sides of both sheets, and performingultrasonic joining to join the first and second sheets together by ascarf joint to form the larger sheet, such that the angle of each sheetintersects the profile of the other sheet to create a first channel inthe first side and a second channel in the second side of the largersheet; and for each channel of the first and second channels—positioning at least one plug in the channel, and performing ultrasonicjoining to join the plug in the channel, and machining a last plug ofthe at least one plug flush with the respective side of the largersheet.
 12. The method of claim 11, wherein each of the first and secondsheets of the material have a thickness of at least 0.07 inch.
 13. Themethod of claim 11, wherein the material is an aluminum material. 14.The method of claim 11, wherein each plug has a thickness of between0.01 inch and 0.04 inch.
 15. The method of claim 11, wherein the plugsare of a different material than the first and second sheets.
 16. Themethod of claim 11, further including, for each channel of the first andsecond channels, positioning at least one additional plug over aninitial plug in the channel, and performing ultrasonic joining to joineach additional plug in the channel until the last plug is proud of asurface of the larger sheet.
 17. The method of claim 16, furtherincluding, for each channel of the first and second channels, machiningthe initial plug ultrasonically joined in the channel in preparation forjoining the additional plug over the initial plug.
 18. The method ofclaim 17, wherein, for each channel of the first and second channels,there is at least one intermediate additional plug prior to the lastplug, and further including, for each channel of the first and secondchannels, machining the intermediate additional plug joined in thechannel in preparation for joining the last plug over the intermediateadditional plug.
 19. The method of claim 11, further includingperforming non-destructive inspection on the larger sheet to determine acharacteristic of the joining.
 20. A method of joining first and secondsheets of a aluminum material having a thickness of at least 0.07 inchto create a larger sheet having a first side and a second side, themethod comprising: for each sheet of the first and second sheets—cutting an angle on a first side of an edge of the sheet, and machininga profile in a second side of the edge of the sheet; overlapping theangles on the first sides of both sheets, and performing ultrasonicjoining to join the first and second sheets together by a scarf joint toform the larger sheet, such that the angle of each sheet intersects theprofile of the other sheet to create a first channel in the first sideand a second channel in the second side of the larger sheet; for eachchannel of the first and second channels— adding plugs until at leastone of the plugs is proud of the respective side of the larger sheet,wherein the adding of the plugs includes— positioning an initial plug inthe channel, with the initial plug having a thickness of between 0.01inch and 0.04 inch, and performing ultrasonic joining to join theinitial plug in the channel, positioning at least one additional plugover the initial plug in the channel, and performing ultrasonic joiningto join the additional plug in the channel, and machining a lastadditional plug flush with the respective side of the larger sheet; andperforming non-destructive inspection on the larger sheet to determine acharacteristic of the joining.